During the production of olefin polymers in a commercial reactor it is often necessary to transition from one type of catalyst system producing polymers having certain properties and characteristics to another catalyst system capable of producing polymers of different chemical and/or physical attributes. Transitioning between similar traditional Ziegler-Natta type catalysts, or compatible catalysts, generally takes place easily. However, where the catalysts are incompatible or of different types the process is typically complicated. For example, transitioning between a traditional Ziegler-Natta type catalyst and chromium based catalyst, two incompatible catalysts, it has been found that some of the components of the traditional Ziegler catalyst or the cocatalyst/activator act as poisons to the chromium based catalyst. Consequently, these poisons prevent the chromium catalyst from promoting polymerization.
In the past, to accomplish an effective transition between incompatible catalysts, the first catalyzed olefin polymerization process is stopped by various techniques known in the art. The reactor is then emptied, recharged and a second catalyst is introduced into a reactor. Such catalyst conversions are time consuming and costly because of the need for a reactor shut-down for an extended period of time during transition.
The term "catalyst killing" or "catalyst killers" refers to the deactivation of the catalyst, which may be a partial or a complete suppression of a polymerization reaction. It is known to use low molecular weight polar gases or polar liquids in order to "kill" traditional Ziegler-Natta catalysts systems. For example, EP-A-116,917 describes using carbon dioxide and alcohol as Ziegler-Natta catalyst killers. U.S. Pat. No. 4,701,489 discloses the use of water to suppress a traditional Ziegler-Natta catalyst polymerization process. It is also known to use high molecular weight products such as polyglycols, epoxides, ethylene copolymers, organic titanium compounds, alkoxysilanes, peroxides, zeolites as a water carrier, or surface active agents to kill traditional Ziegler-Natta catalysts. U.S. Pat. No. 4,460,755 describes a process for converting a polymerization reaction catalyzed by a Ziegler-type catalyst into one catalyzed by a chromium-based catalyst. This particular transition process utilizes a hydroxyl-containing compound which interacts with the Ziegler-type catalyst by a physical or chemical means.
Recently, metallocene type catalyst systems are being employed in polymerization processes to produce polyolefins generally having superior physical and chemical attributes to those products of traditional Ziegler-Natta catalyzed processes.
There are a variety of known metallocene catalyst killers. For example, catalyst killers for metallocene/alumoxane based catalyst systems include methanol and n-butanol. PCT International Publication No. WO 92/14766, published Sep. 3, 1992 describes the use of a volatile and/or non-volatile killers of metallocene based catalysts in a high pressure polymerization process.
It would be highly advantageous to have a process for transitioning between incompatible catalysts, without the need for halting the polymerization reaction, emptying the reactor to rid it of the original catalyst system and restarting the polymerization reaction with another catalyst system.